After printed circuit boards have been manufactured, and before they can be used or placed into assembled products, they must be tested to verify that all required electrical connections have been properly completed and that all necessary electrical components have been attached or mounted to the board in proper position and with proper orientation.
Other reasons for testing are to determine and verify whether the proper components have been used and whether they are of the proper value. It is also necessary to determine whether each component performs properly (i.e., in accordance with the specification). Some electrical components also may require adjustment after installation.
It is possible for humans to manually handle the printed circuit boards for testing, i.e, selecting and delivering the boards to a test fixture, loading the boards into the tester, interacting with the tester by making any required adjustments, removing the boards from the tester, attaching any required repair ticket to the board, and sorting the boards into pass or fail outputs.
There are, of course, several disadvantages and limitations associated with manual handling and probing of the printed circuit boards. For example, the manual probing is a tedious task, and the speed with which a human can perform these tasks is necessarily limited by human abilities. Also, humans sometimes create errors which can be costly, for example, by rejecting a board which is acceptable or by accepting or approving a board which is defective. There are also dangers involved for humans, such as electrical and mechanical hazards. Humans can also create errors by inserting a board into a test fixture with improper orientation (e.g., rotation or translation). There is also danger of static electricity affecting components if the operator is not properly grounded.
As a result of the foregoing problems and limitations, there have been developed fixturing systems for the purpose of handling printed circuit boards for testing. The most common of such fixturing systems is a vacuum fixture. Manufacturers of such type of fixtures include Pylon, Contact Products, Virginia Panel Corp., Teradyne, Gen Rad, Testron, OB Test Group, and others.
There are many disadvantages associated with vacuum fixturing. In vacuum fixturing, atmospheric pressure acts directly on a board with a vacuum beneath it, forcing the board against spring loaded testing probes. Problems arise from the need to maintain a seal around the board. Maintaining a vacuum seal in an automated environment is even more troublesome. Wraped printed circuit boards are commonly encountered and require a separate effort or effect to push and seat them in the fixture gasketing material. Boards with holes or apertures generally complicate vacuum fixturing techniques. Also, node density is limited by atmospheric pressure. The seals and gasketing required also involve much maintenance, and contaminants and other foreign matter may be sucked into the fixture due to the vacuum. There is still some manual handling required with vacuum fixturing, thereby creating opportunities for human error (e.g., improper board insertion, potential for static electricity damage to components if operator is not grounded). Furthermore, it is difficult to do dual sided probing or multi-stage probing (i.e., probing a board with probes of different engagement heights in more than one step) of boards in such fixtures.
Although a mechanical fixturing system has been proposed by Matsushita and others, such systems do have a number of limitations and disadvantages. For example, there are long wire lengths from the tester to the fixture. Also, the conveyor only moves the board to a fixed position at the edge or side of the fixture. Sometimes a special hole, in a defined location, must be provided in the board to accommodate a pin driven by an air cylinder to move the board to a fixed testing position. Further, such systems do not include the capability of being able to move the upper probe plate to a specific vertical position relative to the lower probe plate. Also, such systems are not easily adjusted for different board widths.